Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210907

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
407 AM CDT Fri Jul 21 2017

402 AM CDT

Today through Tonight...

Main forecast concern is with timing rounds of thunderstorms
across the forecast area, and with the potential for severe
weather and additional heavy rainfall especially later tonight.

Early morning surface analysis places a nearly stationary frontal
boundary from roughly the IA/MO border eastward across central IL
and IN. Southwesterly low level jet around 25 kts has driven a
couple of small, slightly elevated MCS`s along the cool side of
the boundary early this morning, primarily affecting the southern
half of the cwa. Expect these thunderstorms to continue to track
to the east-southeast across locations mainly south of a Dixon-
Joliet and Rensselaer line early this morning. While locally heavy
rainfall has occurred with some of these storms (2.00-2.50 inches
per 88D estimation), the storms have been fairly progressive and
this area has seen some of the least rainfall in the cwa over the
past few days, thus no significant precip/flood problems are
anticipated there this morning.

Attention then turns back to our northwest with a mid-level short
wave tracking across the Plains, and associated surface low
pressure which is progged to develop into the Upper Mississippi
Valley by this evening. As this occurs, southeast and eventually
more south surface winds will allow the stationary front to our
south to lift north as a warm front, with 00Z model runs trending
a bit farther north with the boundary into far southern WI by
later tonight. Very warm, moist air mass (temps in 90`s, dew
points in 70`s to near 80) will support strong destabilization
this afternoon once morning MCS debris exits, setting the stage
for scattered thunderstorm development along the northward
shifting warm front. Primary focus for organized strong storms is
expected to by across MN/western WI by mid-late afternoon, with
eventual upscale growth into an MCS expected tonight.

As previously noted, many of the synoptic scale models have
trended farther north with the warm front and also with the
heavier QPF axis with strong storms into tonight. Some of the
high-res guidance continues to bring storms and heavy rainfall
potential across far northern IL overnight, and this appears
reasonable given the effect of lingering clouds/boundaries from
convection early in the day, and low level instability which is
expected to be across the region into tonight. Thus have shifted
qpf axis a bit northward (in agreement with WPC and surrounding
offices), but still have 1-1.50 inch coverage across northern tier
of IL counties. Expect timing of greatest heavy rain threat to be
after midnight, as MCS slowly sags southeast and low level jet
veers more westerly resulting in slower propagation and the
potential for training as Corfidi vectors indicate.

Have made no changes to flash flood watch at this point, though
if models continue to trend in similar fashion could see removal
of southern tier of FFA counties allowed. SPC has enhanced day 1
severe risk just upstream of IL cwa (actually gets into far NW),
with wind- producing bowing segments possible as late day/tonight
MCS moves into the area.

With warm front lifting north-northeastward across the area
today, very warm and humid conditions are expected. If not for
convective cloud debris this morning and residual boundaries/cool
pool from morning storms, would expect to see a few mid-90`s
potentially in the far south/west counties. Have generally kept a
bit of a conservative approach given these limiting factors, as
well as a light southeast wind off the lake farther northeast.
Current forecast touches 105 degree heat index in a couple spots
briefly in warmest part of afternoon, though short duration and
uncertain spatial coverage does not warrant heat headline at this



402 AM CDT

Saturday through Thursday...

Saturday afternoon heat and humidity will again push apparent
temperatures toward or over heat advisory criteria with the support
of southwest flow ahead of an approaching frontal zone.  Model QPF
fields and MSLP trends reflect disagreement in the timing and low
level forcing related to the passage of the front, perhaps due in
part to the potential stabilizing effects from widespread convective
activity expected tonight and in the predawn and morning hours on
Saturday. Still feel that, at the least, isolated to scattered
activity is possible areawide for the Saturday afternoon period,
but toward evening and overnight the focus shifts mostly south of
the Chicago metro area.

A considerable pattern change is then in store, with Sunday looking
like a transition day toward drier weather with cooler temperatures
and lower dewpoints as an upper trough digs over the area and a
broad surface ridge approaches the Great Lakes from the northwest.
Sunday appears to be just a bit cooler and drier than Saturday,
but the change will be more evident on Monday and Tuesday when
some locations do not escape the 70s. By mid to late week the
ridge likely will move far enough east to allow more typical
warmth and moisture to start streaming back into the area, but at
this range we do not appear positioned for a late-week repeat of
the recent very warm heat and high humidity.



For the 06Z TAFs...

Primary aviation forecast concerns are the potential for a few
periods of thunderstorms at/near the terminals through early
Saturday morning.

Early morning surface analysis depicts a surface stationary front
from near the Iowa-Missouri border to the KCMI/KDNV areas of east
central IL. Southwesterly low-level jet of 25 kts depicted in RAP
model analysis is producing lift across the low-level frontal
zone, within a moderately unstable air mass. This was resulting in
an area of thunderstorms extending from eastern IA into parts of
north central IL at 0530Z. Current eastward propagation is also
indicated by several high-res model runs, suggesting that TSRA may
come pretty close to the Chicago terminals after 0730-0800Z this
morning, though some eventual southeastward shift is expected.
Given activity upstream across eastern IA, TSRA could linger
near/south of Chicago terminals into the 11-12Z time frame.

Best focus for additional storms is then expected to develop later
this afternoon/early evening as the stationary front eventually
lifts north as a warm front, in response to surface low pressure
developing into the Upper Mississippi Valley. Fairly low
confidence in details of timing and placement, as morning
convection and any lingering cloud cover/boundaries may impact.
Guidance trends do support the potential for additional late
night convection Friday night.



315 PM CDT

Strong winds from a wake low this morning have dissipated. The baggy
pressure gradient has allowed a lake breeze to form. Despite gusty
southwest winds, the lake breeze is finally moving ashore over the
southwest end of the lake. Variable wind directions are expected
across the lake through tonight.

Winds become southerly tomorrow as high pressure shifts over the
eastern Great Lakes. One to two additional convective systems are
possible through Friday night. Any of these may produce a wake low
that briefly increases winds and waves to values higher than
currently forecast.

A weak low will pass over the lake Saturday night and winds
will become northerly behind it.  A second and possibly stronger
surge of north winds will push down the lake Sunday night into
Monday morning. North winds 15-25 kt are possible. High pressure
moves over the lake Tuesday.



IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     PM Friday to 9 AM Saturday.




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